Visuals: Graphics

Collisions and Chemical Reactions

Not all collisions between reactants result in a chemical reaction. This figure shows four collisions in which the reactants have different orientations; only the first orientation allows for a productive collision. (Unit 12, Section 2)

Potential Energy Diagram

A potential energy diagram showing the potential energies of the chemical species as the reaction progresses. In this case, the carbon in the center has a very high energy because it is temporarily trying to bind to five things rather than its standard four bonds. (Unit 12, Section 3)

Temperature and Activation Energy

The lines on this graph represent the distribution of particles possessing a range of energy values. The orange line represents a low temperature, and the blue line represents a higher temperature. Note that at the higher temperature, a much larger fraction of the molecules will have more than enough kinetic energy to overcome the activation barrier. (Unit 12, Section 3)

Catalysts and Activation Energy

In the presence of a catalyst, the activation energy for the reaction is lower. Note how the two new steps both have activation barriers that are much lower than the original uncatalyzed activation barrier. (Unit 12, Section 5)

Catalytic Converter

Catalytic converters in the exhaust system of a car convert toxic substances such as carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) into gases that are safe to breathe. (Unit 12, Section 5)

The Lactase Enzyme

The lactase enzyme breaks down lactose into simpler sugars. If the body stops producing lactase as we get older, we either have to cut back on the eating of dairy products or take supplements that contain the lactase enzyme that the body no longer makes. Without the enzyme, the reaction to digest lactose is too slow to occur in the body. (Unit 12, Section 5)

Stable vs. Unstable Nuclei

The nucleus on the left is stable because it is small; its diameter is smaller than the range of the strong force. The larger nucleus on the right exceeds the range of the strong force and is therefore less stable. (Unit 12, Section 7)

Decay Chain for Uranium-238

Note how the first step in its overall decay is its rate-determining step because it is so much slower than all of the subsequent steps. Note that every time an alpha particle is emitted, it becomes an element two atomic numbers lower and when a beta particle is emitted, its atomic number moves up by one, without changing its mass number (Unit 12, Section 8)

Radiocarbon Dating

Plants take up radioactive carbon-14 formed in the atmosphere. The radioactive carbon then enters the food chain. So long as a plant or animal is alive, the amount of carbon-14 remains constant. Once the life form is dead, the carbon-14 can only decay, though it takes several hundred years for the decay to be measurable. (Unit 12, Section 8)

Nuclear Binding Energy

A graph of nuclear binding energy for selected nuclei. Note how the nuclear binding energy per nucleon is highest for nickel, which is why nickel and iron are the two most stable elements. (Unit 12, Section 9)

Subcritical Mass and Supercritical Mass

In a supercritical mass, neutrons produced by fission have a high probability of causing subsequent fission reactions. In a subcritical mass, the neutrons escape the material without causing more fission. Every time there is a red branch, more neutrons are formed. Note how the size of the subcritical mass in blue only allows three fission reactions creating neutrons as compared to many more within the blue area for the supercritical mass. (Unit 12, Section 11)